1. Field of the Invention
The present invention relates to a light emitting-diode (LED) and a method for manufacturing the same, and more particularly to a LED comprising a reflective layer including an aluminum layer and an alumina layer so as to improve photoemissive efficiency, which is easily formed using an anode bonding technique, and a method for manufacturing the LED.
2. Description of the Related Art
Generally, light emitting diodes (LEDs) are semiconductor devices made of compounds such as GaAs, AlGaAs, GaN, InGaN, AlGaInP, etc., and are widely used as various light sources in the fields of optical communications and electronic devices.
One of essential factors for determining characteristics of the LED devices is luminance. There have been made many attempts to improve the luminance of LED devices by variously changing their designs.
For example, in order to improve the luminance of LEDs, the LEDs comprise at least one active layer for generating light by recombining electrons with holes. Among these LEDs, a LED comprising active layers, each layer with a thickness of more than 100 Å, is referred to as a multiwell (MW) LED, and a LED comprising active layers, each layer with a thickness of less than approximately 100 Å, is referred to a multiple quantum well (MQW) LED. On the other hand, a LED comprising a single active layer is referred to as a double heterostructure (DH) LED or a single quantum well (SQW) LED. That is, there have been made attempts to improve the luminance of LEDs by adjusting the number and/or the thicknesses of the active layers.
Such conventional techniques have trended toward maximizing light generated by the active layers. However, the aforementioned techniques for improving the luminance of generated light by changing the structure of the active layers create a complicated design of the LED due to increased number of materials, and it is difficult to apply such techniques to a practical process for manufacturing the LED.
In addition to the aforementioned conventional techniques, there has been made an attempt to improve the structure of the LED so as to effectively use light in a uniform quantity, emitted from the active layers.
For example, a reflective layer is formed on the lower surface of a LED, thereby minimizing the amount of light emitted in unnecessary directions, and improving the luminance of light emitted in a desired direction. This technique is embodied by forming a back reflective layer made of a metal with high reflectance such as Al, Ag, or etc., on the lower surface of the LED. FIGS. 1a and 1b illustrate one example of the above conventional technique.
With reference to FIGS. 1a, a LED 10 comprises a substrate 2, a n-type first conductive clad layer 4 formed on the substrate 2, an active layer 6 formed on the first conductive clad layer 4, and a p-type second conductive clad layer 8 formed on the active layer 6. Conventionally, the first conductive clad layer 4 is an n-type layer, and the second conductive clad layer 8 is a p-type layer. Since such configuration is general in LEDs, it is also applied to the present invention. However, the first conductive clad layer 4 may be a p-type layer, and the second conductive clad layer 8 may be an n-type layer. Therefore, selectively, the LED may be grown on a p-type substrate or an n-type substrate. Further, in case the clad layer is formed out of a semiconductor material made of a GaN compound, a sapphire substrate may be used.
As shown in FIG. 1a, the n-type clad layer 4 is electrically connected to a n-type Ohmic contact layer 11 formed at an area of the upper surface of the n-type clad layer 4 without the active layer 6 and the p-type clad layer 8 grown thereon (more specifically, an area of the upper surface of the n-type clad layer 4 exposed to the outside by growing the active layer 6 and the p-type clad layer 8 thereon and selectively removing the grown active layer 6 and p-type clad layer 8), and the p-type clad layer 8 is electrically connected to a p-type Ohmic contact layer 13. A reflective layer 15 made of a metal with high reflectance is formed on the lower surface of the substrate 2.
When voltage is applied to the Ohmic contact layers 11 and 13 of the LED 10, electrons from the n-type clad layer 4 and holes from the p-type clad layer 8 are injected into the active layer 6. Then, the electrons and the holes injected into the active layer 6 are recombined with each other, thereby generating light.
FIG. 1b schematically illustrates light emission from the active layer 6. With reference to FIG. 1b, for example, the light generated from a point (P) of the active layer 6 is emitted in an upward direction (a) of the LED 10, i.e., a direction desired by users, and in a downward direction (b) of the LED 10. Herein, the quantities of light emitted in the two directions are nearly the same.
In the LED 10 of FIG. 1b, the light (b) emitted in the downward direction of the LED 10 reaches the reflective layer 15 disposed on the lower surface of the substrate 2, and is reflected by the reflective layer 15 with high reflectance. Then, a certain quantity of the reflected light (b′) is again emitted in the upward direction of the LED 10 due to the high reflectance of the reflective layer 15 and emitted in the upper direction of the LED 10. Therefore, the reflective layer 15 serves to concentrate the light generated from the active layer 6 so as to emit the light in the upward direction of the LED 10, thereby minimizing the unnecessary loss light.
However, the reflective layer 15 as shown in FIGS. 1a and 1b insufficiently improves the photoemissive efficiency of the LED 10. Particularly, since much of the light emitted toward the reflective layer 15 or reflected from the reflective layer 15 is absorbed by the sapphire substrate 2 with a large thickness, the improvement in photoemissive efficiency of the LED 10 by the reflective layer 15 is not great.
Further, the reflective layer 15 is formed by an additional deposition step after manufacturing the LED 10. Therefore, the entire manufacturing process of the LED 10 is complicated and its production cost is increased. Moreover, it is very difficult to form an Al layer or an Ag layer with excellent adhesiveness on the lower surface of the sapphire substrate using a conventional deposition device.
Therefore, there are required a LED comprising a reflective layer having an improved configuration and being easily manufactured, and a method for manufacturing the LED, thereby more effectively increasing the luminance of the LED.